In general, high-voltage electrical insulating materials for use as insulators and bushings for power transmission lines are of porcelain (ceramics) or glass. Since these insulators are heavy and liable to breakage due to a lack of impact resistance, they require careful handling and impose a burden to workers. In a polluted environment as in seaside areas and industrial areas, there is a tendency that dust, salts and mist attach to the surface of high-voltage electrical insulators, causing leakage currents and dry band discharge leading to flashover failure.
In order to eliminate the drawbacks of ceramic and glass insulators, a number of proposals have been made. For example, U.S. Pat. No. 3,511,698 discloses a weathering resistant high-voltage electrical insulator comprising a member of a thermosetting resin and a platinum catalyst-containing organopolysiloxane elastomer. JP-A 59-198604 corresponding to U.S. Pat. No. 4,476,155 proposes a one-part room temperature curable organopolysiloxane composition which is applied to the outer surface of an electrical insulator of glass or porcelain so that the electrical insulator may maintain its high insulating properties even in the presence of moisture, polluted air, ultraviolet radiation and other outdoor stresses.
JP-A 53-35982 corresponding to U.S. Pat. No. 3,965,065 discloses that a silicone rubber composition with improved electrical insulation is obtained by heating a mixture of an organopolysiloxane capable of heat curing into silicone rubber and aluminum hydrate at temperatures above 100.degree. C. for more than 30 minutes. JP-A 7-57574 corresponding to U.S. Pat. No. 5,519,080 describes that the blending of a methylalkylsiloxane fluid in silicone rubber is effective for providing contact angle recovery with time and preventing flashover failure.
However, the silicone rubber materials used in the prior art techniques mentioned above are not yet fully satisfactory in high-voltage electrical insulation. They must be loaded with large amounts of aluminum hydroxide in order to improve the electrical insulation. This raises a new problem that a high loading of aluminum hydroxide reduces the strength of silicone rubber and renders the silicone rubber brittle. As a consequence, molded parts are often damaged during molding, during attachment and by external factors (weather conditions, human attacks as by gunshots, and bird and animal attacks) after molding.